It is known that the polymerization casting of axially symmetrical articles, such as contact lenses, can be performed by using equipment in which individual molds are arranged in a carousel or in a vertical stack configuration. These individual molds, characterized by an outer cylindrical wall and a mold cavity with an exposed concave bottom surface and containing a liquid polymerizable mixture in the cavity, are caused to rotate about their vertical axis at a rotational speed (and under polymerization conditions) sufficient to create a centrifugal force which causes a radially outward displacement of the liquid reactants in the mold. By maintaining the rotating mold(s) under predetermined and known polymerization conditions, the outwardly displaced liquid reactants are caused to polymerize to a solid polymeric contact lens. The resulting lens is characterized by a convex optical surface which corresponds to the concave surface of the mold and a concave optical surface whose geometry has been created, to a significant degree, by the centrifugal force(s) employed during the polymerization cycle.
In the centrifugal casting of contact lenses on a commercial scale, it is preferred for the attainment of good yield to effect the polymerization or curable reaction under an inert gaseous medium such as argon or nitrogen. This is due to the fact that the oxygen component of air entrained within the polymerization column can inhibit the polymerization reaction and adversely affect the quality and acceptability of the contact lens product. A controlled feed of nitrogen through the polymerization column will purge any entrained air in the polymerization zone and provide an inert environment for conducting the polymerization process.
The aforesaid carousel arrangement is rather complex and quite large with respect to the size of the molds. It requires that each mold be individually rotated on its own separate vertical axis. It is reported that the carousel arrangement suffers from the disadvantages of requiring excess inert gas to eliminate the inhibiting effect of oxygen (in the air) present during the polymerization reaction. The use of excess inert gas during the polymerization of the monomeric reactants causes the entrainment of monomer in the form of vapors and the subsequent deposition and/or polymerization on the walls and equipment. Further information is set forth in U.S. Pat. No. 3,660,545, issued May 2, 1972, the full disclosure of which is incorporated by reference as if set out in full text.
In the vertical stack arrangement a rotatable polymerization tube having an internal circular cross-sectional geometry is adapted to receive at one end of the tube a plurality of circular molds which become seated to one another in the said tube, each mold containing the liquid polymerizable reactants in the mold cavity. In operation, the molds are gravity fed into the upper end of the polymerization tube and free-fall through the tube against an upwardly flowing inert gas, e.g., carbon dioxide, due to their own weight. The exit end of the tube is seated tightly on a revolving plate member which imparts the rotation to the tube and which plate has a centrally defined opening for discharging inert gas into the polymerization tube to contact the descending gravity fed molds. In this type of construction, the revolving plate member would have to be disconnected and displaced from the polymerization column to remove the molds. In addition, the feeding of the inert gas from the center of the revolving plate member into the polymerization column and onto the bottom surface of the bottom most mold could impede the rotation of this mold and thereby prevent the molds within the tube from being rotated at the same speed due to undesirable slippage between the molds and the inner wall of the polymerization column.
This gravity feed arrangement whereby molds are fed into the elongated polymerization zone of the rotating tube suffers from the disadvantage that some of the (sliding) molds may tilt or wobble within the polymerization column such that their horizontal axes form with the longitudinal axis of the tube an angle which is a deviation from the predetermined angle calculated for that particular polymerization run. In the production of small articles requiring high precision and geometry such as contact lenses, heart valves, and the like, the tilted or wobbling molds may result in the production of asymmetrical plastic articles lacking the predetermined optical geometry required in the contact lens article or the high exactness and detail expected in artificial heart valves. In addition, the rotation of the tube by the revolving plate member does not insure that all the molds within the tube will all be rotated at the same speed due to undesirable slippage between the molds and the inner wall of the tube. Consequently, this inability to maintain synchronization of the rotation of the molds with the rotation of the polymerization tube can result in the production of the articles, e.g., contact lenses, disc valves for surgical applications, etc., which fail to fall within exacting predetermined specifications and requirements.
A widely practiced technique for the manufacture of soft contact lenses involves the lathing procedure. This technique has many drawbacks inasmuch as it is a labor intensive operation, requires several steps, is relatively expensive, and the finished lens product is characterized by striations on its optical surfaces. In the lathing technique, an appropriate polymerization medium is first polymerized into a cylindrical shape from which there are cut so-called lens "buttons" or lens blanks, or the lens blanks per se can be made in appropriate molds. The blanks are subjected to a postcure treatment to improve certain of their physical characteristics. A predetermined curved surface is thereafter cut on one face of the blank by using precision lathe machinery and the cut curved surface is polished to an optical surface. Formation of an optical surface on the opposite face of the blank requires adhering the partially cut blank to an arbor or mandrel by means of a waxy substance in a manner that the uncut face of the blank is exposed for the lathing and polishing operations. Thereafter, there are washing and cleaning steps to remove residues from the cutting and polishing procedures and eventually, as with soft contact lenses, soaking in a physiologic solution until osmotic equilibrium is reached at which stage the hydrogel lens attains its final dimensions.